Flex: usage notification emails for reservation thresholds and cap cut-off

This issue is the public record for the GitLab 19.3 release note. Implementation was tracked in internal issues.

GitLab Flex now sends usage notification emails. Billing account managers receive an email when a product reaches 50%, 80%, or 100% of its monthly reservation, and when a capped product reaches its spend cap and is cut off for the rest of the billing period. Each threshold triggers at most one email per product per billing period.

Shipped in GitLab 19.3. Documentation: https://docs.gitlab.com/subscriptions/gitlab_flex/#usage-notifications